1. Field of the Invention
The present invention relates to an hydraulic control device for a working machine, such as a hydraulic excavator.
2. Description of the Related Art
In a typical hydraulic excavator, bleed-off control is performed in which a part of oil discharged from a pump (excess oil) is returned to a tank. This control is generally performed by varying an opening area of a bleed-off passage provided in a control valve for each actuator depending on a control input of an operating unit. Since this passage is provided, each valve is relatively long in a spool-axis direction, and there are disadvantages regarding cost and installation into a machine.
In addition, a structure in which the above-described passage is omitted and a common bleed-off valve is provided for a plurality of control valves is known. In addition, an electronic control method in which the common bleed-off valve is a hydraulic pilot valve and is controlled by a secondary pressure of a proportional solenoid valve controlled by a controller is also known (refer to, for example, Japanese Unexamined Patent Application Publication No. 11-303809).
This method is advantageous in that there is more freedom in control compared to a hydraulic control method in which a pilot pressure corresponding to a control input is directly transmitted to the common bleed-off valve. However, if an abnormality (fail) occurs in a control system, for example, if the proportional solenoid valve malfunctions or disconnection occurs in a signal system which transmits a control signal from the controller to the proportional solenoid valve, the bleed-off valve stops at the maximum opening position and the entire amount of oil discharged from the pump is unloaded, which makes the machine stop completely. As a result, the machine cannot perform any work at the site.